Memories into which data are written permanently in digital form are required for many electronic systems. Such memories are called, inter alia, read-only memories.
Plastic discs, so-called compact discs, which are coated with aluminium are in widespread use as read-only memories for large quantities of data, in particular the digital recording of music. These discs have two different kinds of point-like depressions in the coating, which are assigned to the logic values zero and one. The information is stored digitally in the arrangement of these depressions.
In order to read the data which are stored on a compact disc, the disc is mechanically rotated in a read apparatus. The point-like depressions are scanned via a laser diode and a photocell. Typical scanning rates in this case are 2.times.40 kHz. Approximately 5 Gbits of information can be stored on one plastic disc.
The read apparatus has moving parts which undergo mechanical wear, require comparatively large volumes, allow only slow data access and consume a great deal of power. The read apparatus is furthermore sensitive to vibration and can thus be used only to a limited extent for mobile systems.
Semiconductor-based, in particular silicon-based, read-only memories are frequently used for the storage of smaller amounts of data. When the memory cell arrangement is read, the individual memory cells are selected via a word line. The gate electrode of the MOS transistors is in each case connected to a word line. The input of each MOS transistor is connected to a reference line, and the output to a bit line. An assessment is carried out during the reading process to determine whether or not any current is flowing through the transistor. The logic values zero and one are assigned accordingly.
Technically, the storage of zero and one is brought about in the case of these read-only memories by no MOS transistor being produced, or no conductive connection to the bit line being produced, in memory cells in which the logic value assigned to the state "no current flow through the transistor" is stored. Alternatively, the two logic values can be produced by MOS transistors which have different operating voltages as a result of different implantations in the channel region.
These known silicon memories usually have a planar construction. In consequence, a minimal surface area, which is about 6 to 8 F.sup.2, is required per memory cell, F being the smallest producible structure size in the respective technology. Planar silicon read-only memories are thus limited to memory densities of about 0.9 bit/.mu.m.sup.2 when using 0.4 .mu.m technology.
U.S. Pat. No. 4,954,854 discloses the use of vertical MOS transistors in a read-only memory. The surface of the silicon substrate is for this purpose provided with hole-like trenches against which a source region abuts at the base, against which a drain region, surrounding the trench, abuts at the substrate surface, and along whose flanks a channel region is arranged. The surface of the trench is provided with a gate dielectric, and the trench is filled with a gate electrode. Zero and one are distinguished in this arrangement by no trench being etched and no transistor being produced for one of the logic values. Neighbouring memory cells are insulated from one another by insulating structures arranged laterally with respect to them.